This invention relates to a method and apparatus for testing phosphor and, in particular, is an improvement on the “oil cell” method used for almost fifty years.
The method and apparatus are described in the context of phosphor for thick film electroluminescent (EL) lamp. Any kind of phosphor can be tested with the new apparatus and method.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer can include phosphor particles or there can be a separate layer of phosphor particles adjacent the dielectric layer. The phosphor particles radiate light in the presence of a strong electric field, using relatively little current.
EL phosphor particles are typically zinc sulfide-based materials, including one or more compounds such as copper sulfide (Cu2S), zinc selenide (ZnSe), and cadmium sulfide (CdS) in solid solution within the zinc sulfide crystal structure or as second phases or domains within the particle structure. EL phosphors typically contain moderate amounts of other materials such as dopants, e.g., bromine, chlorine, manganese, silver, etc., as color centers, as activators, or to modify defects in the particle lattice to modify properties of the phosphor as desired. The color of the emitted light is determined by the doping levels. Although understood in principle, the luminance of an EL phosphor particle is not understood in detail. The luminance of the phosphor degrades with time and usage, more so if the phosphor is exposed to moisture or high frequency (greater than 1,000 hertz) alternating current.
The “oil cell” method for testing phosphor typically relies on castor oil as a liquid dielectric in which phosphor particles are dispersed. Suitable electrodes are provided and the phosphor is driven to luminance. Up to a point, the oil cell method provides a quick and convenient way to evaluate new phosphor formulations. The mobility of particles in an electric field in a liquid dielectric is well known; see Lehmann, “Dielectric Behavior of Electroluminescent Zinc Sulfides,” Journal of the Electrochemical Society, Vol. 103, No. 1, pgs. 24–29, January 1956. This mobility is believed to be one of the causes of unstable and non-reproducible measurements of luminance from an oil cell. The oil cell test method has been found to have almost a ±10% variance in measured luminance even under the most controlled conditions with highly experienced operators.
The variability means that life testing is virtually impossible because the luminance changes over time for reasons having nothing to do with the life of the phosphor. (“Life” is generally accepted to mean the time for luminance to decay to half of initial luminance.)
The variability also means that performing a plurality of tests on a sample is essentially pointless. For example, a series of tests on a single sample is essentially the same as a series of tests on a plurality of samples. Any variation in luminance due to a particular parameter is lost in variations from other causes.
Thin, thick film layers of phosphor are known in the art. As used herein, and as understood by those of skill in the art, “thick-film” refers to one type of EL lamp and “thin-film” refers to another type of EL lamp. The terms only broadly relate to actual thickness and actually identify distinct disciplines. In general, thin film EL lamps are made by vacuum deposition of the various layers, usually on a glass substrate or on a preceding layer. Thick-film EL lamps are generally made by depositing layers of inks on a substrate, e.g. by roll coating, spraying, or various printing techniques. A thin, thick-film EL lamp is not a contradiction in terms and such a lamp is considerably thicker than a thin film EL lamp. Other distinctions between the two types of lamps are described in the report Electroluminescent Material for Flat Panel Display, Final Report for CRADA No. ORNL95-0371, October 2000.
U.S. Pat. No. 4,513,023 (Wary) discloses phosphor in a UV curable dielectric layer having a thickness of 0.2–1.2 mils (5.1–30.5 μm). Although phosphor particles in a solid dielectric have less mobility than in a liquid dielectric, the cured layer has a variable thickness across the area thereof, which makes measurements of luminance inconsistent. The option to date, taking phosphor samples and making complete EL lamps, does not guarantee reproducible results and costs considerable time and money.
Thus, the oil cell method and apparatus have been accepted as a quick indication of proof of concept rather than as an analytical tool.
In view of the foregoing, it is therefore an object of the invention to provide a method and apparatus for reproducibly measuring the optical and electrical characteristics of a phosphor.
Another object of the invention is to improve the oil cell test to provide an analytical tool for evaluating phosphor.
A further object of the invention is to provide an oil cell in which the same sample can be tested a plurality of times.
Another object of the invention is to provide an oil cell that can be operated continuously for long periods in order to provide an indication of the operating life of a phosphor.